Load plugins in a deterministic order.

Fixes plugin unit tests on my dev box and is a good idea overall anyway.
Allow run_unit_tests.sh to execute all unit tests in test/unit.

Basically adds __init__.py files to each subdirectory so nose detects these as Python module directories. Also modify dynamic loading of modules in Carl's test cases so the test case can be run with any cwd - including the Galaxy root directory as run_unit_tests.sh will use.

This should roughly double the number of unit tests buildbot runs.